Value MUST be part of code list 'UN/ECE Recommandation 20, including Rec. 21 codes (prefixed with X)'.
/ubl:CatalogueRequest/cac:CatalogueRequestLine/cac:Item/cac:AdditionalItemProperty/cbc:ValueQuantitynot(@unitCode) or (some $code in $clUNECERec20 satisfies $code = @unitCode)